Add a bug button on the prototype? Integrating the bug reporting means you can automatically supply a history of the last n things the user did, or whatever debugging state you think is helpful, and the user will fill in the rest. Similar to automatic crash reporting, except without requiring the crash.

The bug button pops up the usual form for the user to fill out - describe the bug, etc. It's just that more extensive state can be silently added to help recreate the bug, or at least provide the context that is often missing.